A satellite television receiving system includes an "outdoor unit", including a dish-like receiving antenna and a "block" converter, and an "indoor unit" including a tuner and a signal processing section. The block converter converts the entire range of relatively high frequency RF signals transmitted by the satellite to a more manageable, lower range of frequencies.
In a conventional satellite television receiving system for receiving and processing television information transmitted in analog form, the RF signals transmitted by satellite are in the C (3.7 to 4.2 gigaHertz) and Ku (11.7 to 14.2.2 gigaHertz) bands, and are converted by the block converter to the L band (900 to 2000 megaHertz) as a "block". An RF filter section of the tuner of the indoor unit selects the one of the RF signals provided by the block converter corresponding to the selected channel and the selected RF signal is again converted by a mixer/local oscillator section of the tuner to a lower, intermediate frequency (IF) range for filtering and demodulation. Typically, the IF frequency range has a center frequency of 479 megaHertz. Analog satellite television systems typically employ FM modulation, and a baseband video signal is readily obtained from the 479 IF signal by an FM demodulator after filtering by an IF filter. A relatively simple surface acoustic wave (SAW) device can provide adequate filtering.
In newer satellite television systems, such as the DSS.TM. (Direct Satellite System) available from Thomson Consumer Electronics of Indianapolis, Ind., television information is transmitted in digital form. RF signals are transmitted by the satellite in the Ku band, and are converted by the block converter to the L band. The frequency range of the RF signals transmitted by the satellite is somewhat smaller, e.g., between 12.2 and 12.7 gigaHertz, than that for the analog satellite television system, and the frequency range of RF signals produced by the block converter is accordingly somewhat smaller, e.g., between 950 and 1450 megaHertz. As in the analog satellite television receiving systems, the RF signal corresponding to the selected channel has to be reduced in frequency to an IF frequency range for filtering and demodulation. However, the type of filtering ("symbol shaping") required in digital satellite television receiver, cannot readily be performed at the relatively high IF frequency (e.g., 479 megaHertz) employed in an analog satellite television receiver, especially using a SAW device. As a result, a relatively expensive digital filter will be required for filtering the demodulated digital signals. Alternatively, the tuner can employ a second conversion stage to convert the relatively high frequency (e.g., 479 megaHertz) first IF signal to a second, lower frequency (e.g., less than 100 megaHertz) signal for filtering. However, the second conversion stage adds undesirable cost to the receiver.
It is also desirable that the tuner of the digital satellite television receiver be capable of being constructed utilizing components which are already commercially available and therefore relatively inexpensive. Specifically, in this regard it is desirable that the tuner be capable of being constructed utilizing a commercially available integrated circuit (IC) which incorporates a phase locked loop (PLL) for controlling the frequency of the local oscillator. Since a large number of tuner PLL ICs for conventional television receivers which receive and process conventional broadcast and cable television signals are widely available, it is particularly desirable that the tuner of the digital satellite television receiver be capable of being constructed utilizing such a conventional tuner PLL IC.